That is an excellent point about the decongestants. Many consumers do not realize these are often included with an antihistamine.

Actually, though, it appears as though even antidepressants such as Benadryl can create cardiac effects. It is a serotonin-reuptake inhibitor. Therefore, it has the potential to back-suppress dopamine. The dopamine system plays a role in regulating blood pressure (among other functions).

Here's an example of cardiac toxicity from Diphenhydramine (Benadryl). The patient did take a large amount, but still the effect is noteworthy.

http://www.isrjem.org/Sept2003_Diphenydramine.pdf
Abstract
Case Report: A 30 year old female ingesteda lar-uea nlounto f diphenhydramine.
Shortlya fter.s hee xhibiteds eizurea ctivitya nd wide-conrplexta chycardiaS. eizure
activity was successfullytr eatedw ith intravenousb enzodiazepineasn d orotracheal
intubationW. ide-complexta chycardiain itially resolvedu 'ith intravenous odium
bicarbonateb ut laterr ecurredd espitea lkalinizationto a pH of 7.52 and arterial
pCO,28m mHg. The wide complext achycardiare solveds hortlya fier the intravenous
infusiono f phenytoin7 50 mg.
Drug screeningfo r tricyclic overdosea s well as other common toxic ingestantsw as
negative.
Conclusion:P henytoinr nayh aveh ad therapeuticv aluei n this caseo f cardiact oxicity
from diphenhydraminteh atw asr efractoryto alkalinizationw ith sodiumb icarbonate.
lntroduction
Diphenhydraminien gestionr esultingi n toxicity hasb eenr eportedv ery commonly( l).
Therei s somee videnceto suggestth att hec ardiotoxicitoyf diphenhydraminise s imilart o
tricyclica nti-depressa(TntC A)t oxicity.i n thati t hast ype la antiarrhythmpicr opertie(s2 ).
We will presenat casei n which the usualm odalitiesfb r type la antiamhythmitco xicity
wereu nsuccessfuinl treatinga diphenhydramincea rdiotoxicityb ut phenytoinw as.W e
will then review the literaturea nd discusst he use of phenytoini n treatingt ype la
antiarrhythmitco xicity.